1. Field of the Invention
The present invention generally relates to an organic electroluminescent (EL) device (also known as OELD) and a method for fabricating the same, and more particularly, to a white light emitting organic electroluminescent device able to directly emit continuous full color light containing three different frequency bands and a method for fabricating such an organic electroluminescent device easily and with high precision. This invention can effectively simplify the fabrication procedure and improve the luminescence efficiency.
2. Description of the Prior Art
The organic electroluminescent device, upon which C. W. Tang and S. A. Van Slyke (Eastman Kodak Company, Rochester, N.Y.) have made efforts since 1987 to form hetero-structures by employing aluminum trisoxine [a.k.a., tris(8-quinolinol)aluminum] by vacuum evaporation, has attracted tremendous attention due to its advantages over other display panels. These advantages include self-luminescence, large visual angle, short response time, compact size, light weight, reduced dimension in thickness, high brightness, low power consumption, simple fabrication, and the ability for light emitting in a full color range. Therefore, such an organic electroluminescent device is increasingly required to replace the currently used white light sources such as fluorescent lamps and light bulbs so as to save energy, and the technologies thereon have widely been studied in the industry all over the world.
Please refer to FIG. 1, which is a cross-sectional view showing the structure of an organic EL device disclosed in U.S. Pat. No. 4,769,292, issued Sep. 6, 1988, filed Oct. 14, 1987 by Van Slyke et al (Eastman Kodak Company, Rochester, N.Y.), entitled xe2x80x9cElectroluminescent device with modified thin film luminescent zone.xe2x80x9d The organic EL device 10 comprises in sequence: a transparent substrate 11, light transmissive anode 13 formed of tin oxide or indium tin oxide (ITO) by evaporation, an organic hole injecting and transporting zone 15, a luminescent zone 17, and a cathode 19. The luminescent zone 17 is formed by a thin film comprised of an organic host material capable of sustaining hole and electron injection and a fluorescent material (not shown) capable of emitting light in response to hole-electron recombination. When an external voltage is applied to the device 10, the anode 13 injects holes (positive charge carriers) into the luminescent medium 17 while the cathode 19 injects electrons into the luminescent medium 17. The portion of the luminescent medium 17 adjacent the anode 13 thus forms a hole injecting and transporting zone 15. The injected holes and electrons each migrate toward the oppositely charged electrode. This results in hole-electron recombination within the organic luminescent medium 17, which leads to energy released as light according to the chosen fluorescent material.
The fore-mentioned prior art organic EL device has advantages in good quality and enduring lifetime. However, the structure employed can only emit monochromatic lights according to various chosen fluorescent materials, and fail to achieve the objects of emitting white light or continuous full color light.
Please refer to FIG. 2, which is a schematic band diagram showing the structure of an organic EL device disclosed in U.S. Pat. No. 5,668,438, issued Sep. 16, 1997, filed Jun. 6, 1996 and U.S. Pat. No. 5,886,464, issued Mar. 23, 1999, filed Apr. 18, 1997 by Shi et al (Motorola, Inc., Schaumburg, Ill.), both entitled xe2x80x9cOrganic electroluminescent device with emission from hole transporting layer.xe2x80x9d In the EL structure, an anode 22 is formed of tin oxide or indium tin oxide (ITO), an organic hole transporting layer 23 is formed on the anode 22, an organic electron transporting layer 24 is formed on the hole transporting layer 23, and a cathode 25 is formed on the electron transporting layer 24. The materials for the hole and electron transporting layers 23 and 24 are so selected as to satisfy the following inequality:
(EC1xe2x88x92EC2) less than (EV1xe2x88x92EV2)
where EC1 and EV1 respectively represent a conduction band level and a valence band level of the material selected for the hole transporting layer 23; and EC2 and EV2 respectively represent a conduction band level and a valence band level of the material selected for the electron transporting layer 24.
The inequality ensures that the energy barrier for holes to be injected into the valence band of electron transporting layer 24 from the valence band of hole transporting layer 23 is greater than that for electrons to be injected into the conduction band of the hole transporting layer 23 from the conduction band of electron transporting layer 24. In other words, the number of electrons to be injected from the electron transporting layer 24 into the hole transporting layer 23 is much larger than the number of holes to be injected from the hole transporting layer 23 into the electron transporting layer 24. Therefore, electrons and holes recombine in the part of hole transporting layer 23 close to the interface of electron transporting layer 24 and hole transporting layer 13, where light emission occurs. Moreover, in order to facilitate holes to be injected into the hole transporting layer 23 from the anode 22, the EL structure further provides a hole injection layer interposed between the anode 22 and the hole transporting layer 23.
Although the fore-mentioned prior art organic EL device has high electroluminescence efficiency due to light emission from the hole transporting layer 23. However, the structure employed can only emit monochromatic lights according to various chosen fluorescent materials, and fail to achieve the objects of emitting white light or continuous full color light.
In recent years, there are several methods that have been investigated and developed by the industry to realize an organic EL device capable of emitting white light or full color light, including:
1. Color conversion: In this method, a monochromatic light passes through a color conversion material composed of different color conversion layers and is then resolved and converted into light with different colors, e.g. three primary colors, such as red, blue, and green so that an organic EL device capable of emitting white light or full color light can be obtained. However, this method also suffers from a number of problems. First, most of the available color conversion materials are not satisfactory in color purity and luminescence efficiency. Secondly, the background light (such as blue light and UV light) may also be absorbed by the color conversion layers, which often leads to poor contrast and defective pixel quality. And thirdly, the color conversion process is performed by a two-wavelength approach; therefore, chromatic aberration may occur.
2. Color filter: In this method, white light is used as the back-lighting source of the organic EL material. It is useful to achieve full color light when accompanied by LCD color filters. However, the key problem of this method is how to obtain a reliable white light.
3. Three independent colors (RBG): In this method, three primary colors red (R), green (G) and blue (B) are independently demonstrated to realize a full color display or a white light source. However, since the three colors are independently demonstrated, RBG pixels require different driving voltages. A multicolor organic light emitting device thus formed is disclosed in U.S. Pat. No. 5,703,436, issued Dec. 30, 1997, filed Mar. 6, 1996 by Forrest et al (Princeton University, Princeton, N.J.), entitled xe2x80x9cTransparent contacts for organic devices.xe2x80x9d It suffers from complicated fabrication process and larger size. In addition, in such a device, high precision is critically required for the RBG pixels. As shown in FIG. 3, which is a 3-dimensional view showing the structure of an organic EL device disclosed in U.S. Pat. No. 5,952,037, issued Sep. 14, 1999, filed May 8, 1997 by Nagayama et al (Pioneer Electronic Corporation, Tokyo, JP), entitled xe2x80x9cOrganic electroluminescent display panel and method for manufacturing the same.xe2x80x9d The organic EL device comprises: a substrate 30 on which a plurality of first display electrodes 32 corresponding to emitting portions are formed; electrical insulation ramparts 34 projecting from the substrate 30 for exposing at least portions of the first display electrodes 32 respectively; organic function layers 36 each including at least one organic electroluminescent medium formed on exposed portions of the first display electrodes 32; second display electrodes 38 formed on the organic function layers 36; and each electrical insulation rampart having an overhanging portion 385 projecting in a direction parallel to the substrate preferably at an upper part thereof. It is found that the fabrication process may be difficult and complicated. On one hand, the RBG pixels formed of three different organic EL materials are employed may have different luminescence efficiencies, lifetimes, driving conditions. For example, the red light shows poor purity and may shift to orange color. The red light also has shorter lifetime and may adversely affect the overall performance of the display. On the other hand, the method is performed by a two-wavelength approach; therefore, chromatic aberration may occur.
Therefore, the present invention has been made to solve such problems in view of the forgoing status and to further provide a method for fabricating a white light emitting organic electroluminescent (EL) device able to directly emit continuous full color light containing three different frequency bands so as to realize white light emitting.
It is the primary object of the present invention to provide a white light emitting organic electroluminescent device, incorporating a luminescent layer, an electron transporting layer, and dopant materials to realize continuous full color light containing three different frequency bands with better color uniformity and improve the luminescence efficiency.
It is another object of the present invention to provide a white light emitting organic electroluminescent device so as to realize white light with better full color display quality.
It is still another object of the present invention to provide a white light emitting organic electroluminescent device, incorporating a bias voltage, instead of three independent driving voltages, to emit three primary colors (red, green and blue), so as to simply the fabrication process, downsize the device and reduce the cost.
It is still another object of the present invention to provide a white light emitting organic electroluminescent device to replace the currently used white light sources such as fluorescent lamps and light bulbs so as to save energy.
It is still another object of the present invention to provide a white light emitting organic electroluminescent device, incorporating a doping technique to realize continuous full color light containing three different frequency bands with better color uniformity and improve the luminescence efficiency.
In order to achieve the foregoing objects, the present invention provides a white light emitting organic electroluminescent device, comprising: a substrate; an anode formed on said substrate; at least one hole transporting layer formed on said anode; at least one luminescent layer formed on said hole transporting layer, wherein a first dopant is doped into said luminescent layer; at least one electron transporting layer formed on said luminescent layer, wherein a second dopant is doped into said electron transporting layer; and a cathode formed on said electron transporting layer; wherein a first light is emitted by said first dopant, a second light is emitted by said second dopant, and a third light is emitted by said luminescent layer when the device is applied with a bias voltage.
The present invention further provides a method for fabricating a white light emitting organic electroluminescent device, comprising the steps of: providing a substrate; forming, in sequence from substrate up, an anode, at least one hole transporting layer, at least one luminescent layer, at least one electron transporting layer, and a cathode; doping a first dopant into said luminescent layer; and doping a second dopant into said electron transporting layer.
Other and further features, advantages and benefits of the invention will become apparent in the following description taken in conjunction with the following drawings. It is to be understood that the foregoing general description and following detailed description are exemplary and explanatory but are not to be restrictive of the invention. The accompanying drawings are incorporated in and constitute a part of this application and, together with the description, serve to explain the principles of the invention in general terms. Like numerals refer to like parts throughout the disclosure.